Per https://fedoraproject.org/wiki/Releases/26/ReleaseBlocking , the cloud_base qcow2 and raw_xz images are release-blocking for Fedora 26. However, they have been missing from composes after 20170429.n.0, apparently because the compose for aarch64 started failing after that compose. Currently the images build fine on x86_64 but fail on aarch64, and because of this, even the x86_64 images are not included in the compose by Pungi. This is an automatic Beta blocker: "Bugs which entirely prevent the composition of one or more of the release-blocking images required to be built for a currently-pending (pre-)release" - https://fedoraproject.org/wiki/QA:SOP_blocker_bug_process#Automatic_blockers
I believe https://koji.fedoraproject.org/koji/taskinfo?taskID=19708655 means this is ON_QA - if I'm reading that right, Dennis has adjusted things such that the task is considered 'completed' if x86_64 works but aarch64 does not, which should mean this should be resolved for the next compose.
aarch64 and x86_64 both work now. so any future RCs should include both.
The blocking images are confirmed to be present in Beta-1.3 and Beta-1.4, so we can close this as fixed.